The present invention relates to a digital broadcast receiver for receiving a digital broadcast signal transmitted as a digitally modulated video signal.
A usual satellite broadcast receiver is a receiver for receiving an FM (frequency modulated) satellite broadcast signal transmitted with a frequency modulated video signal and as shown in FIG. 13, a desired frequency modulated signal is frequency converted from a first IF (intermediate frequency) signal from a digitally modulated signal into a designated second intermediate frequency signal, a frequency modulated signal in the desired channel is taken out at a channel filter and then it is frequency demodulated.
A local oscillation signal used for the frequency conversion is generated at a PLL (phase-locked loop) synthesizer 30, which is composed of a local oscillator 31, a prescaler 32 frequency dividing a local oscillation signal, a programmable frequency divider 33, a phase frequency detector 34, a reference frequency signal divider 35, a reference frequency signal generator 36 and a loop filter 37. The local oscillation frequency Fvco is expressed by (eq. 1). EQU fvco=(fr/R).times.Npsc.times.Np (eq. 1)
where fvco: oscillation frequency of the local oscillator,
fr: frequency of the reference frequency signal, PA1 Npsc: frequency dividing ratio of the prescaler, PA1 Np: frequency dividing ratio of the programmable frequency divider, and PA1 R: frequency dividing ratio of the reference frequency signal. PA1 channel selection means inputting said digitally modulated signal of an RF band and for frequency-converting a desired digitally modulated signal into an IF signal; PA1 oscillation means for generating a local oscillation signal used for a frequency conversion at the channel selection means; PA1 oscillation frequency control means for controlling a frequency of a local oscillation signal of the oscillation means; PA1 phase noise characteristic control means for improving the phase noise characteristic of the local oscillation signal generated at the oscillation means; PA1 filter means for extracting the IF signal corresponding to the selected channel; PA1 orthogonal detection means for orthogonally detecting the IF signal extracted at the filter means; PA1 AID conversion means for converting an analog output of the orthogonal detection means into a digital signal; and PA1 digital demodulation means for demodulating the digital output of the AID conversion means; and wherein PA1 the phase noise characteristic of the local oscillation signal is improved.
The local oscillation signal according to the frequency of the desired channel is obtained by varying the dividing ratio of programmable frequency divider 33 by microcomputer (CPU) 50. AFC (automatic frequency control) for compensating the frequency drift of the first IF signal due to the frequency drift of a frequency converter (not shown in the drawing) in a receiving antenna for satellite broadcast is made by that microcomputer 50 controls the frequency dividing ratio of programmable frequency divider 33 in PLL synthesizer 30 by a frequency drift detecting signal outputted from a FM demodulator 201, as disclosed in U.S. Pat. No. 1,871,000.
A usual receiver for digital broadcast receiving a transmitted signal using QPSK (quaternary phase-shift keying) modulation as a digital modulation shown in FIG. 14 is a receiver for FM satellite broadcast frequency-modulating a video signal and a QPSK-modulated PCM (pulse code modulation) audio signal and a desired FM signal is obtained by frequency-converting into a designated second IF signal from the first IF signal outputted from the antenna for satellite broadcast and an FM signal of the desired channel is taken out through a channel filter and is frequency modulated and then it is separated into a video signal and an audio signal and they are processed.
A local oscillation signal used for the frequency conversion is generated at a PLL synthesizer 30, which is composed of a local oscillator 31, a prescaler 32 frequency dividing a local oscillation signal, a programmable frequency divider 33, a reference frequency signal generator 36, a reference frequency signal divider 35, phase detection means 34 and a loop filter 37. The local oscillation frequency can be expressed by (eq. 1) like the case of broadcast receiver receiving a previously-mentioned FM signal.
Therefore, a local oscillation signal corresponding to a desired channel frequency is obtained by varying a frequency dividing ratio of the programmable frequency divider by microcomputer 50.
The control by microcomputer 50 is necessary for a control of many LSIs (large scale integrations) such as not only PLL synthesizer 30 but also PCM sound signal processor 205 and the like. A common bus such as an IIC (inter integrated circuit) bus is used as a control bus because the number of output terminals of microcomputer 50 has a limitation.
AFC for compensating the frequency drift of the first IF signal due to the frequency drift of a frequency converter in a receiving antenna for satellite broadcast is made by that microcomputer 50 controls the frequency dividing ratio of programmable frequency divider 33 in PLL synthesizer 30 by a frequency drift detecting signal outputted from a FM demodulator 201, as disclosed in U.S. Pat. No. 1,871,000, like an example of the prior art. The control data of the microcomputer is renewed at every several tens milliseconds, considering malfunction of a PLL synthesizer and each LSI (large scale integration circuit) due to a surge of thunder even after signal reception.
However, in the case in which a usual AFC is done at a PLL synthesizer for channel selection, the reference frequency fr/R of a phase detection at the PLL synthesizer determines AFC accuracy and if the frequency dividing ratio of the reference frequency signal is so much decreased, the number of steps of AFC control becomes large and a correct AFC can not be done. In a receiver of digital modulation such as QPSK modulation, because the phase noise characteristic of a local oscillation frequency signal influences a bit error rate, the phase noise must be small and it is necessary not only to improve a phase noise characteristic of a local oscillator circuit itself but also to improve a phase noise characteristic by the PLL synthesizer such as making a reference frequency fr/R of the PLL synthesizer for phase frequency detection large.
In the case of a receiver receiving a digitally modulated signal such as QPSK modulation, when a control data is inputted from a microcomputer to a PLL synthesizer, the control data gives an interference to the local oscillator circuit for channel selection and the noise characteristic of the local oscillation signal rapidly deteriorates. Thus, the noise characteristic of the local oscillation signal gives an influence to the bit error rate.
By using a control bus from the microcomputer to the PLL synthesizer in common with a control bus of the other signal processing LSIs, when the other signal processing LSIs are controlled during reception, the control data is inputted also to the PLL synthesizer and whenever AFC during reception is made in the PLL synthesizer or the control data is periodically renewed against malfunction of the PLL synthesizer due to a surge or the like, noise in the local oscillation signal becomes big and deteriorates the bit error rate and the quality of reception. This is a problem.